Not too long ago I posted this Raytraced Reflections Without Temporal Anti Aliasing thinking that I’d found a fix but after encountering fireflies and noise again it seems like that fix was more context specific and that there’s a much broader issue that extends beyond just reflections.
The bottom line is that when rendering a sequence out of the Movie Render Queue you can sometimes get fireflies and noise particularly in the reflections. This of course is a common issue with 3D rendering in general beyond Unreal Engine but usually there’s solutions to mitigate or remove them either within the rendering process or in post.
Currently the only “fix” that seems to eradicate all fireflies and noise in the reflections as far as I know is leaving on TAA in the Anti Aliasing section when rendering. If you’re still using spatial or temporal samples TAA will still need to be enabled for both the fireflies and noise to be gone, this of course is not recommended for various reasons but aside from the issues that TAA can introduce such as ghosting, it has a noticeable downgrade in image quality which is a big reason to even use these multisampling anti aliasing features in the first place so it’s kind of counterintuitive.
So far I have experienced a lot of noise in the reflections which is unaffected by the amount of samples I use whether that be from the spatial/temporal samples or directly increasing the reflection samples per pixel itself. Disabling or enabling the raytracing denoisers also seems to have little effect on the noise.
r.RayTracing.GlobalIllumination.FireflySuppression exists for global illumination and it seems to work for removing the fireflies for the most part. I’ve also come across fireflies in the World Depth when rendering out different passes. Another thing to note is that disabling the Tone Curve will make the fireflies significantly worse in most cases and when bloom is enabled it picks up the fireflies and noise in the reflections which leaves you with a lot of bright flickering.
I don’t get fireflies occurring on every render but only some and the seemingly random nature of when I get them makes trying to systematically reproduce them difficult. Hopefully I can bring awareness to this because its quite a big deal when working with Unreal Engine for cinematics!